Many textile machines wherein textile yarns travel longitudinally along their length to be subsequently wound require a device for monitoring the velocity or the length of the traveling textile yarn then wound. The result is used, for example, to correct deviations in velocity and to obtain as accurate as possible information regarding the length of the yarn which has been wound as of any given time.
For example, in connection with bobbin winding machines producing cheese-type bobbins, or yarn packages, there is often a requirement that all produced bobbins have exactly the same yarn length if possible. This is primarily necessary if these bobbins are to be subsequently placed on a creel and drawn off together to form a warp beam. Different yarn lengths lead to residual yarn of different lengths on the bobbin tubes in such a case. With yarn material of high quality this results in unacceptable, and possibly costly losses.
A widely used method to determine the yarn length on such bobbin winding machines is to count the revolutions of the bobbin or of the drive roller for the bobbin and to determine the wound-on amount of yarn using calculations based on the circumference of the bobbin or of the drive roller for the bobbin. Since the circumference of the drive roller is constant, the determination of the circumferential velocity poses no problems. Nevertheless, the slippage which typically occurs between the drive roller and the bobbin can be a considerable source of errors. The resultant calculated velocity or yarn length value may be greatly distorted since, to avoid so-called "pattern winding," a slippage between the drive roller and the bobbin is intentionally generated during the entire bobbin travel or at least in so-called pattern zones in which the bobbin diameter and the diameter of the drive roller have a defined relationship to each other.
Measuring the number of bobbin revolutions is relatively simple. However, the exact determination of the progressively changing diameter (and in turn the bobbin circumference) occurring during the course of bobbin winding can be problematical. If the angle of rotation of the bobbin support is used as the measurement for the bobbin radius, considerable errors can also result because of deviations in the pressure of the bobbin on the drive roller.
A number of methods are known for determining the yarn velocity by contact with the yarn. Such a method increases the yarn tension and is unsuitable for higher re-spooling velocities because of the inertia of the element which is moved along with the yarn.
To avoid the mentioned disadvantages it has been proposed in European Patent Publication EP 0 000 721 A1 to determine the yarn velocity by means of two contactless operating sensors placed at a fixed distance from each other. Optically or capacitively operating sensors, for example, are suited for this purpose. These sensors determine stochastic yarn signals in the form of analog noise signals resulting from irregularities of the yarn surface or yarn mass in the longitudinal direction of the traveling yarn. The stochastic signal detected upstream in the direction of travel is temporally displaced sufficiently far until it shows a maximal similarity with the stochastic signal detected at the sensor placed downstream, indicating the time delay existing between the two signals. The delay of the first signal determined in the course of this operation corresponds to the length of time the yarn requires to travel from the first to the second sensor. Since the distance between the two sensors is known, it is possible in this way to easily determine the yarn velocity. However, the mathematical operations, which are customarily called a cross-correlation method, are subject to a certain expenditure of time. This poses no problems if the yarn is subjected to no or only very small accelerations. However, more rapid velocity changes, such as those which occur in the winding process because of common patterning disturbances, for example, cannot be handled in such a way that an exact measurement can take place. Since differentiation of the cross-correlation function is required for determining the primary maximum of this function, the measured value data scatter increases with low signals in relation to the noise acting as an interference value.